1. Field of the Invention
This invention pertains to nanopost arrays containing millions of nanosize posts per square centimeter and a process for making them from nanochannel templates.
2. Description of Related Art
Micromagnetic microscopy has been critical to both the physics and applications of micromagnetism. With the recent advent of new techniques, such as magnetic force microscopy and magnetic scanning near-field optical microscopy, the need for reliable and inexpensive submicron magnetic standards has grown stronger than ever before. Fabrication of large-scale nanopost arrays of ferromagnetic posts or wires and their application as magnetic standards and others is presented herein. Nanopost glass arrays made from the glass templates each containing more than several million nanometer-scale cylindrical channels that are parallel and uniform in diameter. The geometric placement of the channels in the template can be controlled to a high degree. The nanopost glass arrays can be utilized as nanosize magnetic standards, especially for dimensions smaller than 50 nm where no such standards, it is believed, are currently available. Also, the nanopost arrays can be used in any other application where parallel disposition and diameter uniformity of the nanoposts is desired.
Nanopost glass arrays have been fabricated previously by various techniques. To date, most of the non-template techniques are still in the developmental stage, are complicated, and expensive. Of the current non-template techniques, electron beam lithography is the most widely used since it offers control and planar pattern flexibility. Electron beam lithography, however, suffers serious disadvantages, including the high complexity and cost. Compared with parallel processes, such as the new technique discussed herein, the serial writing electron beam technique is many orders of magnitude slower. In addition, only very limited field, smaller than about 100.times.100 microns may be patterned each time, and because of thermal drift and mechanical vibration, it is virtually impossible to coherently stitch multiple fields together to fabricate very large periodic structures containing submicron features.
Closely related to electron beam lithography is X-ray lithography. As a parallel process, X-ray lithography is relatively fast. Nevertheless, because it uses electron beam lithography to write its masks, X-ray lithography faces the same problems that electron beam lithography suffers. Furthermore, X-ray sources and masks can be quite expensive. In fact, to date, interference lithography, i.e., the interference of two coherent beams of light to define a pattern, has been the only parallel technique capable of fabricating large, periodic arrays or gratings of nanoposts, or more accurately, nanowires, with high aspect ratios over 100 in a reasonably short time. All of the arrays produced by interference lithography, however, have been just one-dimensional, with high-aspect-ratio wires lying parallel to each other in a plane.
Previous alternative techniques for fabricating nanopost arrays using templates have also been limited by the lack of good templates. The templates commonly used included polycarbonate track-etched membranes and anodized aluminum, both of which suffer several disadvantages compared to the nanochannel glass templates. Polycarbonate track-etched membranes are made by shooting heavy ions through a dielectric material creating nuclear damage tracks. These tracks can be preferentially etched away to give hollow pores. The pores of the polycarbonate track-etched membranes, and hence their corresponding grown nanoposts, are non-parallel, non-uniform in diameter, placed randomly, often not even cylindrical, and occasionally cross paths in a high density structure.
Anodized aluminum templates are made by anodic oxidization of aluminum. Though better than polycarbonate track-etched membrane nanoposts, anodized aluminum nanoposts are only approximately parallel and cylindrical, and non-uniform in diameter.
Recently, carbon nanotubes have been used as templates or hosts to grow metallic nanoposts or nanowires by capillary action. Nanowires fabricated by this method, however, are curly, non-uniform in diameter, randomly distributed, and mostly discontinuous. In short, the non-uniformity of the previous templates has led to non-uniformity in the resulting ferromagnetic nanopost arrays, which consequently are not applicable for use as magnetic standards.
The nanochannel glass templates used herein provide a host matrix enabling simultaneous large scale complex patterning of nanometer sized elements or posts or wires with the accuracy and precision exceeding that of ion and electron beam writing. Such a glass template can contain or have a packing density of as many as about 10.sup.12 posts/cm.sup.2 of varying configurations with the post diameters varying from less than 10 nm to micron size with aspect ratio of up to about 10,000. These glass templates have a thickness of about 2 microns to 5 mm, typically 20-500 microns, and temperature stability from about -272.degree. C. to about +600.degree. C.
The nanochannel glass template can be fabricated by starting with an elongated acid etchable glass rod, typically 25-50 mm in diameter and up to 100 cm long that is inserted into a hexagonal tube of inert glass that yields a snug fit. The cone/clad rod is fused under vacuum during a drawing process and drawn to fine filaments of 0.03-0.5 cm and cut to lengths of 30-90 cm. The filaments are stacked into bundles nearly 3.5 cm flat to flat, drawn and restacked and redrawn until the desired tube diameter is achieved. Just before the final draw, a cladding can be added for strength and to facilitate handling by placing the bundle inside a hollow tube and drawing under vacuum. To relieve stress, the glass is annealed after each draw. The glass is then cut to the desired lengths, polished and etched in a weak acid, such as 1% HCl or HNO.sub.3 or acetic acid, to remove the etchable glass used and leave behind the clad nanochannel glass template. The Tonucci et al U.S. Pat. No. 5,264,722 gives more details pertaining to fabrication, more complex structures and other aspects of the nanochannel glass templates.